1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device using a light emitting element, particularly a thin film electroluminescent (EL) element, as the light source.
2. Description of the Related Art
The liquid crystal display device is an apparatus for displaying by controlling the orientation state of liquid crystal molecules, and is usually provided with a back light in order to improve the visual recognition and enable use in a dark place. In the conventional liquid display device, the back light was generally composed as follows.
(a) A plurality of cold cathode ray tubes arranged linearly and with a diffusion film disposed immediately beneath the liquid crystal element. PA1 (b) Cold cathode ray tube is disposed at one end of a resin plate for leading light. PA1 (c) The so-called dispersion type EL element having a resin film diffused fluorescent material enclosed by electrodes is disposed immediately beneath the liquid crystal element. PA1 the light source is formed by sequentially stacking a metal electrode, an insulating layer, a phosphor layer, an insulating layer, and a transparent electrode on a glass substrate, and PA1 the glass substrate is bonded to the opposite side surface of the side surface of the liquid crystal layer of either one of the pair of light permeable substrates through an insulation layer. PA1 the light source formed by sequentially stacking a transparent electrode, an insulating layer, a phosphor layer, an insulating layer, and a metal electrode is disposed in a region other than the display region of the liquid crystal display element, on the surface of the opposite side substrate of the display surface side of the pair of light permeable substrates. PA1 the light source is formed by sequentially stacking a metal electrode, an insulating layer, a phosphor layer, an insulating layer, and a transparent electrode, and PA1 the light source is disposed in a region other than display picture element of the liquid crystal display element, on the liquid crystal layer side surface of the display surface side substrate of the pair of light permeable substrates.
FIG. 12 is a sectional view showing a constitution of a liquid crystal display device 1 using the back light of (b) above. The liquid crystal display device 1 comprises a liquid crystal element part 6 including a liquid crystal 5 between a pair of light permeable substrates 2, 3, and sealing the substrates 2, 3 with a sealing agent 4, and a back light part 10 containing a light source 8, a light conducting plate 7 and a reflector 9. At the side of one substrate 3 of the liquid crystal element part 6, the light conducting plate 7 is disposed, and at the side of one end 7a of the light conducting plate 7, the light source 8 is disposed. On the opposite side of the light conducting plate 7 and light source 8 from the liquid crystal element part 6, the reflector 9 is disposed. The light emitted from the light source 8 which is realized by a linear light source such as cold cathode ray tube is led up to immediately beneath the liquid crystal element part 6 by the light conducting plate 7, and is reflected by the reflector 9 and enters the liquid crystal element part 6.
FIG. 13 is a sectional view showing a constitution of a liquid crystal display device 11 using the back light (c) above. In the liquid crystal display device 11, the back light part 10 is composed of a dispersion type EL element 17 which is a plane light source. The dispersion type EL element 17 is composed by interspacing a light emitting layer 16 between a substrate 12 on which a transparent electrode 14 is formed, and a substrate 13 on which a metal electrode 15 is formed. Of the substrates 12, 13, at least the substrate 12 is light permeable. The light emitting layer 16 is formed by dispersing fluorescent material 16b such as zinc sulfate (ZnS) powder doping copper (Cu) or silver (Ag) in the binder 16a of synthetic resin, low melting glass powder or the like. When a voltage is applied between the transparent electrode 14 and the metal electrode 15, the light emitting layer 16 emits light, and the produced light enters the liquid crystal element part 6 directly and after being reflected by the metal electrode 15.
Such a dispersion type EL element 17 is weak in humidity. Generally, the brightness half-period of dispersion type EL element is 2,000 to 4,000 hours, and the light emission life is short, and when used as the back light, frequent maintenance is required.
Incidentally, reducing the thickness and enhancing the performance are demanded in the display used as a man-machine interface along with the progress of computer, and as one of such measures it is proposed to integrate the liquid crystal display element and back light in the liquid crystal display device. Requirements of the back light for integration with the liquid crystal display element are the homogeneous composition with the liquid crystal display element, plane light source, and long life.
When integrating the back light and liquid crystal display element for reducing the thickness and enhancing the performance, in the back light by using the cold cathode ray tube of (a) or (b), since it is not of the same composition as the liquid crystal display element composed of members possessing light permeability such as glass substrate, it is impossible to reduce the thickness by integrating with the liquid crystal display element. In the back light using the dispersion type EL element (c), although it is possible to reduce the thickness by integrating with the liquid crystal display element, the light emission life is short, and the life of the liquid crystal display device is limited by the life of the back light.